System for generating a beam of acoustic energy from a borehole, and applications thereof

ABSTRACT

In some aspects of the invention, a device, positioned within a well bore, configured to generate and direct an acoustic beam into a rock formation around a borehole is disclosed. The device comprises a source configured to generate a first signal at a first frequency and a second signal at a second frequency; a transducer configured to receive the generated first and the second signals and produce acoustic waves at the first frequency and the second frequency; and a non-linear material, coupled to the transducer, configured to generate a collimated beam with a frequency equal to the difference between the first frequency and the second frequency by a non-linear mixing process, wherein the non-linear material includes one or more of a mixture of liquids, a solid, a granular material, embedded microspheres, or an emulsion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/166,842, filed Jul. 2, 2008, the contents of which are incorporatedherein by reference.

GOVERNMENT RIGHTS

This invention was made with Government support under CooperativeResearch and Development Agreement (CRADA) Contract NumberDE-AC52-06NA25396 awarded by the United States Department of Energy. TheGovernment may have certain rights in this invention.

FIELD

The present invention relates generally to acoustic interrogation ofrock formations around a borehole and more particularly to using thecombination of an acoustic source including a single transducer or anarray of transducers in the wellbore coupled to a non-linear materialfor producing an acoustic beam as a probing tool from a borehole tointerrogate the properties of rock formations and materials surroundingthe borehole.

BACKGROUND

Acoustic interrogation of subsurface features tends to be limited by thesize and power of practical sources, and in practice, the output ofdownhole acoustic transducers is limited by the power transmissioncapabilities of the wire line cable. High frequency signals have arelatively short penetration distance, while low frequency signalsgenerally require large sources, clamped to the borehole wall, tomaximize energy transfer to the formation and minimize unwanted signalswithin the well bore. It is difficult to generate a collimated acousticbeam signal in the 10 kHz-100 kHz range from the borehole to probe therock formation surrounding a borehole with conventional low-frequencytransducers. Conventional low-frequency acoustic sources in thisfrequency range have low bandwidth, less than 30% of the centerfrequency, and very large beam spread that depends on the frequency,such that as the frequency decreases, the beam spread increases. Sharpfocus requires a number of conditions to be satisfied, including a longsource array, uniform coupling of all the transducers to the rockformation around the borehole and knowledge of the acoustic velocitiesof the rock formation. In the borehole environment, these conditions arenot often achievable because of underlying physics constraints,engineering feasibility or operating conditions.

Acoustic beam sources based on a non-linear mixing of acoustic waveshave been proposed for general applications in fluid media, such asunderwater sonar, since the 1950s. For subsurface applications, U.S.Pat. No. 3,974,476 to Cowles discloses an acoustic source for boreholesurveys. The disclosure of Cowles describes an acoustic sourcegeneration device that is not physically possible in a borehole of thetypical size used by the oil and gas industry. For example, thegeneration of a 1 kHz frequency beam by mixing two frequencies around 5MHz in a borehole environment violates basic physical principles. Atypical wireline logging tool has a diameter of 3⅝ inch (9.2 cm), thusthe wavelength of a 1 kHz wave in a typical fluid of 1500 m/s will be1.5 m. This represents close to 10 times the borehole diameter. This 1kHz acoustic wave cannot stay collimated without violating the basicuncertainty principle of wave diffraction physics. Moreover, the mixingof 5 MHz frequencies to generate a 1 kHz wave represents a step-downfrequency ratio of 5000:1, which has not been demonstrated to beachievable in practice. The dimensions of the Cowles proposed toollength of 4.5 m is too long and impractical to fit in present daylogging strings.

SUMMARY

In accordance with an aspect of the invention, a device, positionedwithin a well bore, configured to generate and direct an acoustic beaminto a rock formation around a borehole, is disclosed. The devicecomprises a source configured to generate a first signal at a firstfrequency and a second signal at a second frequency; a transducerconfigured to receive the generated first and the second signals andproduce acoustic waves at the first frequency and the second frequency;and a non-linear material, coupled to the transducer, configured togenerate a collimated beam with a frequency equal to the differencebetween the first frequency and the second frequency by a non-linearmixing process, wherein the non-linear material includes one or more ofa mixture of liquids, a solid, a granular material, embeddedmicrospheres, or an emulsion.

In accordance with an aspect of the invention, a method of generating abeam of acoustic energy in a rock formation penetrated by a borehole isdisclosed. The method comprises generating a first acoustic wave at afirst frequency; generating a second acoustic wave at a second frequencydifferent than the first frequency, wherein the first acoustic wave andsecond acoustic wave are generated by a transducer located within theborehole; transmitting the first and the second acoustic waves into anacoustically non-linear medium to produce a collimated beam by anon-linear mixing process, wherein the collimated beam propagatesthrough the non-linear medium in a same direction as an initialdirection of the first and second acoustic waves and has a frequencyequal to a difference of the first and the second acoustic waves,wherein the non-linear material includes one or more of a mixture ofliquids, a solid, a granular material, embedded microspheres, or anemulsion; and directing the collimated beam in a given direction awayfrom the wellbore into the rock formation.

In accordance with an aspect of the invention, a method of generating abeam of acoustic energy in a rock formation penetrated by a borehole isdisclosed. The method comprises generating a first acoustic wave at afirst frequency; generating a second acoustic wave at a second frequencydifferent than the first frequency, wherein the first acoustic wave andsecond acoustic wave are generated by a transducer located within theborehole; transmitting the first and the second acoustic waves into anacoustically non-linear medium to produce a collimated beam by anon-linear mixing process, wherein the non-linear medium includes one ormore of a mixture of liquids, a solid, a granular material, embeddedmicrospheres, or an emulsion; directing the collimated beam in a givendirection into the rock formation; and receiving the collimated beam atone or more receivers after it has reflected or backscattered from aninhomogeneity in the formation, materials near the borehole, or both.

These and other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious Figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a generalized diagram of the device for producing thecollimated beam in accordance with an aspect of the invention.

FIGS. 2 a, 2 b and 2 c show different modes of generating a differencefrequency through a non-linear process in accordance with aspects of theinvention.

FIGS. 3 a and 3 b show comparisons of experimental results andtheoretical predictions of the non-linear mixing in water in terms ofthe amplitude of the collimated beam and axial (z-direction) position.

FIGS. 4 a and 4 b show experimental results of the non-linear mixing inwater as the non-linear medium in terms of the amplitude of thecollimated beam at various excited frequencies and axial (z-direction)and lateral (x-direction) positions.

FIGS. 5 a and 5 b show an aspect of the invention where the collimatedbeam is produced by a chirp burst.

FIGS. 6 a, 6 b and 6 c show an aspect of the invention where thecollimated beam is produced by a chirp burst.

FIGS. 7 a, 7 b and 7 c show an aspect of the invention where thecollimated beam produced by the non-linear mixing process using a CNCfoam block.

FIG. 8 shows an aspect of the invention where the collimated beamproduced by the non-linear mixing process using a CNC foam blocktransmitted through an enclosed aluminum pipe.

FIG. 9 shows an aspect of the invention where the collimated beamproduced by the non-linear mixing process using the 310M ceramic blockas the non-linear material.

FIG. 10 shows an aspect of the invention where the device is used tocharacterize formations and/or materials near the borehole.

FIG. 11 shows the corresponding axes of rotation of the acoustic beamguide in accordance with an aspect of the invention.

FIG. 12 shows an aspect of the invention where the collimated beam,produced by the non-linear mixing process using the 310M ceramic blockas the non-linear material, penetrates a metal pipe casing.

FIGS. 13 a and 13 b show an aspect of the invention where the collimatedbeam after steering with an acoustical mirror exits the metal pipecasing.

FIG. 14 shows an aspect of the invention where the device is used withor without an acoustic focusing system to look straight down a borehole.

FIGS. 15 a, 15 b and 15 c show an experimental set-up and results of theimaging of an object outside the pipe in accordance with an aspect ofthe invention.

DETAILED DESCRIPTION

FIG. 1 shows a generalized diagram of the device for producing thecollimated beam in accordance with an aspect of the invention. In someembodiments, one or more sources 110 are used to produce a first signalat a first frequency and a second signal at a second frequency. By wayof a non-limiting example, the signals can be produced by a 2-channelsignal generator. Similar signal or function generators may be used. Thesignals from the sources are received by one or more signal amplifiers120 and are transmitted to one or more transducers 130 that are used togenerate acoustic waves at the first and the second frequencies.Piezoelectric transducers are one type suitable for this application. Ifmore than one transducer is use, they can be arranged in an arrayconfiguration. By way of non-limiting examples, the array configurationcan be linear, circular, a filled circle or a square array. Thetransducers within the array are divided into two groups, wherein thefirst group of transducers is driven by a source at the first frequencyand the second group of transducers is driven by the source or by adifferent source at the second frequency. In some aspects of theinvention, the source configured to generate the first frequency and thesource configured to generate the second frequency drive all thetransducers simultaneously. By way of a non-limiting example, the firstfrequency is 1.036 MHz and the second frequency is 0.953 MHz. In someembodiments, the first frequency and the second frequency is between 300kHz and 2 MHz.

The acoustic signal is transmitted in a non-linear material 140 togenerate a collimated acoustic beam by way of a non-linear mixingprocess. The non-linear material can be a liquid, a mixture of liquids,a solid, a granular material embedded in a solid casing, embeddedmicrospheres, or an emulsion. By way of a non-limiting example of such anon-linear material is 310M ceramic foam sold by Cotronics of Brooklyn,N.Y., which is composed of over 99% pure fused silica ceramic andprovides low thermal expansion and conductivity, high thermal shockresistance and high thermal reflectance. 310M has a density of 0.80g/cm³ and a speed of sound of 1060 m/s. Another non-limiting example ofthe non-linear material is a urethane foam board material. This type offoam is typically used for Computer Numerically Controlled (hereinafter,“CNC”) machining. The CNC foam has a density of 0.48 g/cm³ and a speedof sound of 1200 m/s. Depending on the operating conditions in theborehole, other non-linear materials can be used as a non-linear mixingmedium with suitable low sound velocity, high non-linear coupling,absorption length, shock wave length, temperature and pressure operatingranges, as well as, other requirements required by operabilityspecifications. Moreover, the length of the non-linear material can bevery compact and can range from between 5 cm to 2 meters depending onthe type of materials being used.

This non-linear behavior may be characterized through the analysis ofthe properties of P-waves resulting from the non-linear mixingphenomenon in which two incident waves at two different frequencies, f₁and f₂, mix to generate third frequency components at the harmonics andintermodulation frequencies f₂−f₁,f₂+f₁, 2f₁ and 2f₂, etc. In an aspectof the invention, the non-linear collinear mixing phenomenon is designedto occur in the non-linear material inside the wellbore. In general,only the resulting third wave of difference frequency f₂−f₁ is ofinterest to this application. The higher frequencies only propagate ashort distance and tend to be absorbed in the non-linear materialitself. In some embodiments, the third wave or collimated beam has afrequency between 10 kHz and 100 kHz.

The collimated beam is received by one or more receivers 150 located ineither the same borehole where the collimated beam is produced oranother borehole. For example, the receivers can be an acoustictransducer, a hydrophone or another type of receiver suitable for thefrequency range of interest. The received signal can be filtered byband-pass filter 160 and amplified by a pre-amplifier 170. The filteredand amplified signal can be displayed on a digitizer, such as a digitaloscilloscope 180. The digital oscilloscope 180 can be controlled by acomputer 190. The computer 190 can also be used to control the signalgenerator 110.

FIGS. 2 a, 2 b and 2 c show different modes of generating the differencefrequency in a non-linear material. The notations f, f₁ and f₂ refer tohigh frequency signals. The signals received from the source 110 and thepower amplifier 120 by a transducer 210, enter a non-linear material220. After a certain propagation length, the difference frequency isgenerated in the non-linear material 220. FIG. 2 a shows the generationof a difference frequency f₂−f₁ by applying two different signals havingtwo different frequencies f₁ and f₂ to the same transducer 210. FIG. 2 bshows the generation of a difference frequency Δf by applying anamplitude modulated signal of frequency f and a modulation of Δf. FIG. 2c shows the generation of a difference frequency f₂−f₁ by applying twodifferent signals having a first frequency f₁ to a first transducer 230and a second frequency f₂ to a second transducer 240. The high frequencybeams overlap in the non-linear material and produce the differencefrequency f₂−f₁.

In accordance with the above, and by way of a non-limiting example, thefirst frequency is 1.036 MHz and the second frequency is 0.953 MHz. Thecollimated acoustic beam generated by the interaction with thenon-linear material will have a frequency equal to the differencebetween the first frequency and the second frequency. In this example,the collimated acoustic beam has narrow frequency band with a cleardominant frequency of 83 kHz. In some embodiments, the collimatedacoustic beam can have a relatively broad frequency range, wherein thefirst frequency is a single, narrow band frequency and the secondfrequency is swept across a broader range of frequencies. The firstfrequency may also be swept across a broad range of frequencies as wellas the second frequency. In either case, the first frequency, the secondfrequency, or both can be a coded signal or an uncoded chirp. Onebenefit of coding the signal is signal to noise ratio improvement.

In some embodiments, the collimated beam is encoded with a time-varyingcode, which can be introduced into either the first or the secondsignal, or both. The time-varying code may include one or more of avariation in amplitude, a variation in frequency, and/or a variation inphase of the first, the second, or both the first and the secondsignals. The received time-varying code of the collimated beam can beused to measure a time-of-flight of the beam. Additionally, in someembodiments, the collimated beam can be broad-band if one of the primaryfrequencies is swept through a range of frequencies while the other isfixed. Thus, the resulting third beam f₂−f₁ will be swept across a widefrequency range.

FIG. 3 shows results of laboratory measurements in relation totheoretical predictions based on non-linear mixing and wave propagationtheory. Acoustic waves are distorted by the nonlinear characteristics ofthe medium through which they propagate. The nonlinear propagation ofacoustic waves can be modeled via the Khokhov-Zabolotskaya-Kuznetsov(KZK) equation, which can be solved by a finite difference approximatescheme. The KZK equation explains various nonlinear characteristics suchas diffraction of sound pressure, attenuation of sound pressure (i.e.,absorption), and generation of a harmonic frequency component (i.e.,non-linearity), and models the shape of an acoustic signal as a soundpressure given such parameters as initial transmission sound pressure,transducer diameter and transducer array geometry, propagated distance,and medium. The KZK non-linear parabolic equation takes into account thecombined effects of diffraction, absorption, and non-linearity indirective sound beams. The KZK equation for an axisymmetric sound beamthat propagates in the positive z direction can be expressed in terms ofan acoustic pressure p as follows:

$\begin{matrix}{\frac{\partial{\,^{2}p}}{{\partial z}{\partial t^{\prime}}} = {{\frac{c_{0}}{2}( {\frac{\partial{\,^{2}p}}{\partial r^{2}} + {\frac{1}{r}\frac{\partial p}{\partial r}}} )} + {\frac{D}{2c_{0}^{3}}\frac{\partial{\,^{3}p}}{\partial t^{\prime 3}}} + {\frac{\beta}{2\rho_{0}c_{0}^{3}}\frac{\partial{{}_{}^{}{}_{}^{}}}{\partial t^{\prime 2}}}}} & (1)\end{matrix}$where t′=t−z/c₀ is a retarded time variable, t is time, c₀ is a smallsignal sound speed, r=(x²+y²)^(1/2) is a radial distance from the z axis(i.e., from the center of the beam), ∂²/∂r²+(1/r)∂/∂r is the transverseLaplacian operator, and ρ₀ is the ambient density of the fluid.Furthermore,

$D = {\rho_{0}^{- 1}\lbrack {( {Ϛ + \frac{4\eta}{3}} ) + {\kappa( {\frac{1}{c_{v}} - \frac{1}{c_{p}}} )}} \rbrack}$is the sound diffusivity of a thermoviscous medium, where ζ is the bulkviscosity, η the shear viscosity, κ the thermal conductivity, and c_(v)and c_(p) the specific heats at constant volume and pressure,respectively. The coefficient of non-linearity is defined by β=1+B/2A,where B/A is the parameter of non-linearity of the medium. The firstterm on the right-hand side of equation (1) accounts for diffraction(focusing) effects, the second term for absorption, and the third termfor non-linearity of the attenuating medium. Further details on the formand use of the KZK model may be found in Y.-S. Lee, “Numerical solutionof the KZK equation for pulsed finite amplitude sound beams inthermoviscous fluids,” Ph.D. Dissertation, The University of Texas atAustin (1993), which is hereby incorporated by reference in itsentirety.

For the laboratory measurement, the transducer was excited at 0.953 MHzand 1.036 MHz leading to a collimated beam having a frequency equal tothe difference of 1.036 MHz−0.953 MHz=83 kHz. The collimated beam wasproduced by the non-linear mixing process using water as the non-linearmaterial. FIG. 3 a shows the amplitude of the generated beam for a rangeof z and x positions of a hydrophone receiver. FIG. 3 b shows a plot ofthe observed axial intensity profile, in good agreement with theory.

FIG. 4 a shows the results obtained by exciting the transducers at avariety of different frequencies, and thus producing the collimated beamhaving a different frequency. The results are shown as a plot ofamplitude as measured by a voltage, versus a position along the z-axisdirection measured in millimeters. In this laboratory test, collimatedbeams were produced having at frequencies of 10 kHz, 37 kHz, 65 kHz, 83kHz and 100 kHz. As can be seen in the figure, the collimated beams havesimilar beam profiles along the z-axis direction. FIG. 4 b shows thebeam cross section at a distance of 110 mm from the emitter. In thisfigure, the amplitude of the beam as represented by a voltage is plottedagainst the x-axis direction as measured in millimeters. The resultsindicate that the collimated beam at a variety of frequencies showssimilar highly concentrated beam cross sections in the x-direction,unlike waves of the same frequency that would be more spread out in thex-direction.

As discussed above, the collimated beam can have a relatively narrowfrequency range, wherein the one or more transducers are excited by asource producing a particular frequency, or the collimated beam can havea relatively broad frequency range. An example of the production of thecollimated beam having a relatively broad frequency range is shown inFIGS. 5 a and 5 b. By way of a non-limiting example, FIG. 5 a shows achirp signal of finite duration that has a frequency ranging from 900kHz to 1 MHz and a burst of a frequency of 1 MHz. FIG. 5 b shows theresultant burst plotted as an amplitude as represented in voltage versustime in microseconds.

FIG. 6 a shows a series of lateral scans at selected distances in thez-direction from the transmitter of the beam shown in FIG. 6 b. Theselected distances are 10 cm, 20 cm, 30 cm, 40 cm, 50 cm and 60 cm. Theplot of amplitude as determined by voltage versus x-axis distance showsthat the beam spread is small and relatively constant and independent ofdistance in the z-direction from the transducer. A frequency spectrum ofthe collimated beam is shown in FIG. 6 c. The figure shows that theusable frequency range for this particular arrangement is from 20 kHz to120 kHz. The low end of the usage frequency range can be as low as 5 kHzand is only limited by the size of the borehole. Other frequency bandsmay be used for the collimated beam including the acoustic loggingfrequencies that are typically in the kHz range and the boreholeteleviewer-type band that are typically in the hundreds of kHz to MHzrange. One benefit of such an arrangement is that the use of a widebandwidth chirp signal source in a borehole would tend to result in animproved signal to noise ratio in comparison with a non-chirped source.The chirped signal further may allow for an improved time-delayestimation that would be beneficial in imaging applications.

FIG. 7 a shows the collimated beam produced by the mixing process usingthe CNC foam block as the non-linear material. A transducer array 710 isconfigured to produce acoustic wave at frequencies of 1.000 MHz and1.087 MHz. The transducer array 710 is coupled to the CNC foam 720 wherethe two acoustic signals mix forming a collimated beam 730 having afrequency of 87 kHz. The CNC foam block has an 80 mm aperture from whichthe collimated beam propagated. FIG. 7 b shows the amplitude of thecollimated beam in the time domain at a lateral distance of 90 mm(x-axis) and an axial distance of 20 mm (z-axis). FIG. 7 c shows thecollimated beam in the frequency domain having a strong peak at 87 kHz.

FIG. 8 is similar to FIG. 7 a, but shows the collimated beam 810generated by the transducer array 820 and CNC foam block 830 arrangedwithin an enclosure 840. As shown, the enclosure 840 is an aluminum pipehaving an overall length of 323 mm, an internal diameter of 140 mm andan exterior diameter of 153 mm.

FIG. 9 is similar to FIG. 7 a and shows the collimated beam produced bythe non-linear mixing process using the 310M ceramic block as thenon-linear material. A transducer array 910 is configured to produceacoustic signals at frequencies of 1.353 MHz and 1.440 MHz. Thetransducer array 910 is coupled to the 310M ceramic block 920 where thetwo acoustic signals mix forming a collimated beam 930 having afrequency of 87 kHz. The 310M ceramic block 920 has a 110 mm aperturefrom which the collimated beam propagated. As can be seen in the figure,the collimated beam has side lobes that extend into the near fieldregion at around a few centimeters from the aperture of the ceramicblock; however, these side lobes do not extend into the far field regionof the beam.

FIG. 10 shows an aspect of the invention where the device is used tocharacterize formations and/or materials near the borehole. One or moresources 1005 produce signals at a first and a second frequency. Thesignals are transmitted to a signal amplifier or amplifiers 1010 thatare configured to increase the power of the signals. The signalsmodified by the amplifier 1010 are transmitted to one or moretransducers 1015 that are configured to generate acoustic waves at thefirst and the second frequency. The acoustic waves are transmitted to anon-linear material 1020, which mixes the waves by way of the mixingprocess to produce a collimated acoustic beam 1025.

The collimated acoustic beam 1025 can be steered in a particulardirection by an acoustic beam guide 1030. The acoustic beam guide 1030can be an acoustic reflector or an acoustic lens. The acoustic reflectorcan be a material with different acoustic impedance from the surroundingmedium in which the beam propagates. One non-limiting example of such anacoustic reflector is metal plate. The acoustic lens is configured tofocus the collimated acoustic beam at a particular focal point anddirection and can have a concave shape. A Fresnel-type mirrorarrangement can also be used for the acoustic beam guide. The acousticbeam guide can be rotated into a particular orientation by use of one ormore actuators 1035 coupled to the guide, as shown in more detail inFIG. 11. In some embodiments, the acoustic beam guide 1030 may not beused, and the collimated beam would propagate along the axis of theborehole.

The collimated beam 1040 can be reflected off the guide 1030 and steeredto a particular direction toward an object of interest 1045 near theborehole. Inhomogeneities of the formations, such as object 1045 or anadjacent bed located along the beam will generate reflection orscattering of the acoustic beam. The reflected and scattered waves 1050are received by one or more receivers 1055 in the same borehole (for thecase of single well imaging) or in another borehole (for the case ofcross-well imaging). The receivers 1055 can be coupled to the guide1030, so that the receivers are configured to receive the reflectedwaves 1050 as the guide 1030 moves. The signals received by thereceivers 1055 can be transmitted to processing electronics 1060 foranalysis. The processing electronics 1060 can include a computer withappropriate software for characterizing the rock formation, includingproducing 2D or 3D images of the formation. The downhole instrumentationis housed in an enclosure 1065 to permit standard well loggingoperations.

In some aspects of the invention, the entire device including thetransducers 1015, the non-linear material 1020 and receivers 1055 can bemoved up and down the length of the borehole to image a particularformation near the borehole. Moreover, the entire device with or withoutthe receivers 1055 can be rotated around the axis of the borehole toimage formations in any azimuthal direction around the borehole.

FIG. 11 shows the corresponding axes of rotation of an acoustic beamguide 1105. The direction of the collimated beam is steered byselectively controlling the azimuth of the guide by rotation around theguide axis 1110, and the inclination 1115, the angle between the planeof the front of the guide and the guide axis. By use of actuators (notshown) the plane of the guide can be effectively controlled in azimuthand inclination. The actuators can thus be used for steering or changingthe direction of the collimated beam.

FIG. 12 shows the collimated beam, produced by the non-linear mixingprocess using the 310M ceramic block as the non-linear material,penetrating a metal pipe casing. A transducer array 1205 is configuredto produce acoustic signals having frequencies of 1.000 MHz and 1.087MHz, for example. The transducer array 1205 is coupled to the 310Mceramic block 1210 where the two acoustic signals mix forming acollimated beam 1215 having a frequency of 87 kHz, which propagatesthrough the metal pipe casing 1220. The transducer array 1205 can berotated around the longitudinal axis of the borehole to image aformation around the borehole. The reflected or backscattered beam fromthe formation can be received by one or more receivers (not illustrated)in the borehole or in another borehole. The receivers can be coupled tothe transducer array 1205 to rotate in a similar manner such that thereflected or backscattered beam is received by the receivers. As can beseen in the figure, the beam maintains its collimation after exiting themetal pipe casing 1220.

FIGS. 13 a and 13 b show the collimated beam after steering with anacoustical mirror and exiting the metal pipe casing. FIGS. 13 a and 13 bare similar to FIG. 12, with the difference that the non-linear material(water in this case) is producing the non-linear beam along the pipe andthe beam is steered out of the pipe perpendicular to the initialpropagation direction with the help of an acoustical mirror plate. Atransducer array 1305 is configured to produce acoustic signals having afrequency of 0.953 MHz and 1.036 MHz, for example. The transducer array1305 is coupled to a non-linear material (water) 1310 where the twoacoustic signals mix forming a collimated beam 1315 having a frequencyof 83 kHz, which reflects from the acoustical mirror 1320 and propagatesthrough the metal pipe casing 1325. As can be seen in the figure, thebeam maintains its collimation after exiting the metal pipe casing 1325,and can be easily steered by rotating the acoustical mirror in such away that the angle of incidence of the collimated beam is changed. FIG.13 b shows the beam steering that results when the mirror 1320 has beenrotated.

FIG. 14 shows an aspect of the invention where the device is used withan acoustic focusing system. One or more sources 1405 produce signals ata first and a second frequency. The signals are transmitted to a signalamplifier or amplifiers 1410 that are configured to increase the powerof the signals. The signals modified by the amplifier 1410 aretransmitted to one or more transducers 1415 that are configured togenerate acoustic signals at the first and second frequencies. Theacoustic signals propagate to a non-linear material 1420, which mixesthe signals by way of the mixing process to produce a collimatedacoustic beam 1425.

In some embodiments, the collimated acoustic beam 1425 is incident on anacoustic focusing system 1430. The collimated beam tends to have acertain beam spread, which increases as the beam propagates through theenclosure (i.e., pipe). This beam spread means that at a certaindistance from the beam origin, the beam will interact with the walls ofthe enclosure, which tends to produce undesirable effects. The acousticfocusing system 1430 reduces this interaction of the beam and theenclosure walls by focusing the beam, and thus reducing the beam spread.The focusing need not reduce the beam profile to a point, but merelyproduce a well defined beam that is not distorted or attenuated due tothe reflections from the walls of the enclosure, such that the beamprofile does not spread too much angularly. One non-limiting example ofthe acoustic focusing system 1430 is a Fresnel lens made of Plexiglassor other materials that when appropriately shaped reduces the beamspread. The acoustic focusing system 1430 can include a variety ofmaterials including a chamber filled with a liquid of different soundspeed than the non-linear material in the enclosure, where the chamberis properly shaped, either convex or concave depending on the liquidsound speeds. In general, any material that is reasonably matched inacoustic impedance with that of the non-linear material in the enclosurecan be used as the acoustic focusing system 1430.

In some embodiments, the acoustic focusing system 1430 is not used whenthe beam 1425 produced by the non-linear mixing in the material 1420 issufficiently well-defined and does not spread too much angularly. Inthis case, the beam 1425 exits the material 1420 without having beenfurther modified.

A housing or enclosure 1435 is configured to house and support thetransducers 1415, the non-linear material 1420, the acoustic focusingsystem 1430, and one or more receivers 1440. The focused acoustic beamis directed along the axis of the housing 1435 and is reflected orscattered from an object of interest 1445. The object 1445 can includeinhomogeneities in the rock formation such as invaded zones, the cementbond with casing, damaged zones, fractured zones, stratigraphic layering(particularly at high apparent dip, i.e., for high angle wells inrelatively low dip formations). The receivers 1440 are configured toreceive the reflected or scattered signal 1455 and the signal isprocessed by processing electronics 1450.

FIGS. 15 a, 15 b and 15 c show an experimental set-up and results of theimaging of an object outside of the pipe in accordance with an aspect ofthe invention. FIG. 15 a shows the experimental set-up that is similarin design to FIG. 10, wherein a source transducer 1505 is configured togenerate acoustic signals and is coupled to a non-linear material 1510that is configured to produce a collimated acoustic beam 1515 by anon-linear mixing process. The source transducer 1505 can be driven by asource generator and a power amplifier (both not shown). An enclosure1520, such as a pipe, is configured to house the transducer 1505, thenon-linear material 1510, as well as, an acoustic beam guide 1525, andone or more receivers 1530. The collimated acoustic beam 1515 isdirected out of the enclosure 1520 by the acoustic beam guide 1525. Byway of non-limiting example, in this arrangement, the acoustic beamguide 1525 is an acoustic reflector. The reflected collimated beam 1530is incident on an object 1535 outside of the enclosure 1520. The object1535 can include inhomogeneities in the rock formations such as invadedzones, the cement bond with casing, damaged zones, fractured zones,stratigraphic layering (particularly at high apparent dip, i.e., forhigh angle wells in relatively low dip formations). The collimated beam1540 is received by the one or more receivers 1550 (either located inthe same borehole or in another borehole) after is has reflected orbackscattered from the object 1535.

In the experimental set-up of FIG. 15 a, the object was rotated 360°about an axis 1545 and measurements were made of the sound intensity asrecorded by receivers 1550. In this set-up, the object 1535 was a solidblock of aluminum with a slightly irregular shape, placed approximately61 cm from the pipe wall. Both the pipe and the block were immersed inwater. FIG. 15 b shows a polar plot of the measured reflected intensityand FIG. 15 c shows a polar plot of the measured reflection time. Inboth FIGS. 15 b and 15 c, the cross-section of the aluminum block isshown for comparison with the measured data. As shown in FIG. 15 b,there is a large signal when the face of the block is in a positionmaximizes the reflected signal at the receiver. Thus, each peakrepresents a face of the block. FIG. 15 c shows the time-of-flight. Asthe block is rotated, the faces come forward and recede, changing thetotal distance the sound beam has to propagate. It is understood that inthe borehole configuration, the intensity image will be obtained byrotation of the device. Thus the amplitude of the reflected signalrepresents reflections from inhomogeneities around the perimeter of theborehole.

The recordings of the received waveforms are processed to generate animage of the reflection or transmission characteristics of theformation. The propagation direction of the beam and the time-of-flightmay fix the locations where scattered waves are generated,distinguishing this device from normal sonic imaging techniques usingconventional non-directional monopole and dipole sources. An associatedeffect of using a beam compared with conventional sources is that thecomputation of an image of formation acoustic properties may not requirea detailed specification of the rock formation's velocity field. Thepropagation direction of the beam and the time-of-flight measurementsimplify and improve the ability to identify the location where thewaves are reflected or scattered. In particular, the knowledge of theorientation of the beam exiting the tool localizes the sources ofrecorded scattered waves along the beam direction, and the time delaylocalizes the position of scattered sources along the beam path. Thus,the borehole imaging with a beam source presents a simplification andreduction in uncertainty of the final time image in contrast toconventional (not beam) sources which require an accurate detailedvelocity model for computation of the 3D image. Furthermore, because thebeam is focused and steerable, in azimuth and inclination with respectto the borehole, the imaging would tend to have higher resolution thanobtained with a conventional (not beam) source. The method allows fordetailed imaging of features including invaded zones, cement bondingwith casing, damaged zones, fractured zones, stratigraphic layeringparticularly at high apparent dip (the angle between the plane of thebedding and the plane perpendicular to the tool axis). The broad banddifference beam frequency for the invention ranges from 1 kHz to 100kHz. The low end of this frequency range, also used by some conventionalsonic logging tools, achieves a depth of penetration up to one hundredfeet. It is important to note that, since the beam is broadband and canbe encoded, the signal to noise ratio of the detected signal would beconsiderably enhanced after processing and decoding. Because of thebroadband beam characteristics with greater depth of penetration andhigher signal to noise ratio due to encoding, the method also allows fordetailed imaging and/or characterization of non-linear properties ofrock formation and its fluid contents surrounding the borehole.

The various configurations described in detail above are for thepurposes of illustration only. Modifications to the configurations canbe made for other applications without departing from the invention. Forexample, in the Logging While Drilling (LWD) and pipe conveyedconfigurations, using technology that allows the tool to pass throughthe bottom of the drill string, the compact acoustic beam generationdevice will enable efficient look ahead of the bit resulting in thedetection of over-pressured zones or significant changes in the rheologyof the formation before they are reached by the drill-bit. Steering ofthe beam also enables the indirect measurement of the dip and azimuth ofreflecting bodies ahead of the bit. Another application is the detectionof fault geometry ahead of the bit.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be a variety ofuseful embodiments, it is to be understood that such detail is solelyfor that purpose and that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover modificationsand equivalent arrangements that are within the spirit and scope of theappended claims. For example, though reference is made herein to acomputer, this may include a general purpose computer, a purpose-builtcomputer, an ASIC including machine executable instructions andprogrammed to execute the methods, a computer array or network, or otherappropriate computing device. As shown in FIGS. 10 and 14, the datacollected by the receivers would undergo some processing and are eitherstored in memory in the tool, or transmitted up hole for furtherprocessing and storage. As a further example, it is to be understoodthat the present invention contemplates that, to the extent possible,one or more features of any embodiment can be combined with one or morefeatures of any other embodiment.

1. A system for generating a beam of acoustic energy in a borehole, thesystem being configured and sized to be positionable within the boreholeand comprising: a transducer configured and arranged to generate a firstacoustic wave at a first frequency and a second acoustic wave at asecond frequency; a non-linear material arranged to receive the firstand second waves, wherein, in operation of the transducer, the first andsecond acoustic waves pass through the non-linear material, and acomposition of the non-linear material produces a collimated beam bymixing of the first and second acoustic waves, such that the collimatedbeam has a frequency equal to a difference of the frequencies of thefirst and the second acoustic waves, and wherein the collimated beam hasa frequency of at least 20 kHz; and a steering device arranged to directthe collimated beam away from the borehole into the material around theborehole.
 2. The system in accordance with claim 1, further comprisingone or more receivers arranged to receive the collimated beam after ithas reflected or backscattered from an inhomogeneity in the materialaround the borehole.
 3. The system in accordance with claim 2, whereinthe one or more receivers are located in the borehole.
 4. The system inaccordance with claim 3, wherein the one or more receivers are locatedin another borehole.
 5. The system in accordance with claim 1, whereinthe transducer includes a plurality of transducers arranged in an array.6. The system in accordance with claim 1, wherein the steering device isselected from the group consisting of an acoustic reflector, an acousticlens, and combinations thereof.
 7. The system in accordance with claim2, wherein the transducer, the acoustically non-linear medium, thesteering device, and the receivers are arranged within an enclosure. 8.The system in accordance with claim 3, further comprising a processor incommunication with a memory having machine executable instructionsstored therein which, when executed cause the processor to analyze thecollimated beam after it has reflected or backscattered from aninhomogeneity in the material around the borehole to generate an imageof the material.
 9. The system in accordance with claim 8, furthercomprising an encoder arranged to encode the collimated beam with atime-varying code by introducing a time varying component including oneor more of chirping or frequency sweep to one of the first and thesecond acoustic waves, wherein the analysis comprises using the encodingto measure a time-of-flight of the collimated beam.
 10. The system inaccordance with claim 9, wherein the time varying components areselected from the group consisting of a variation in amplitude, avariation in frequency, a variation in phase, and combinations thereof.11. The system in accordance with claim 4, further comprising aprocessor in communication with a memory having instructions storedtherein which, when executed are arranged to analyze the collimated beamafter it has reflected or backscattered from an inhomogeneity in thematerial around the borehole to generate an image of the materialbetween the boreholes and to generate information to characterize linearand non-linear properties of the material and fluid contents surroundingthe borehole.
 12. The system in accordance with claim 11, furthercomprising an encoder arranged to encode the collimated beam with atime-varying code by introducing a time varying component including oneor more of chirping or frequency sweep to one of the first and thesecond acoustic waves, wherein the analysis comprises using the encodingto measure a time-of-flight of the collimated beam.
 13. The system inaccordance with claim 12, wherein the time varying components areselected from the group consisting of a variation in amplitude, avariation in frequency, a variation in phase, and combinations thereof.14. The system in accordance with claim 4, further comprising aprocessor in communication with a memory having instructions storedtherein which, when executed are arranged to analyze the collimated beamafter it has reflected or backscattered from inhomogeneities in thematerial around the borehole to generate images selected from the groupconsisting of invaded zones, cement bonding, damaged zones, fracturedzones, stratigraphic layering, sources of scatter, and combinationsthereof.
 15. The system in accordance with claim 1, wherein thecollimated beam of at least 20 kHz has a frequency range.
 16. The systemin accordance with claim 15, wherein the frequency range of thecollimated beam is between 20 kHz and 120 kHz.
 17. The system inaccordance with claim 1, wherein the length of the nonlinear material isbetween 5 cm and 2 m.
 18. The system in accordance with claim 1, whereinthe first frequency has a range of frequencies.
 19. The system inaccordance with claim 1, wherein the second frequency has a range offrequencies.
 20. The system in accordance with claim 1, wherein thematerial around the borehole is rock formation, cement, or casing, orcombinations thereof.
 21. The system in accordance with claim 1, whereinthe non-linear material is selected from the group consisting of: amixture of liquids, a solid, a granular material, embedded microspheres,an emulsion, and combinations thereof.
 22. A system of generating a beamof acoustic energy in a borehole, the system being configured and sizedto be positionable within the borehole and comprising: an array oftransducers configured and arranged to generate a first acoustic wave ata first frequency and a second acoustic wave at a second frequency; anon-linear material arranged to receive the first and second waves,wherein, in operation of the transducer, the first and second acousticwaves pass through the non-linear material, and a composition of thenon-linear material produces a collimated beam by mixing of the firstand second acoustic waves, such that the collimated beam has a frequencyequal to a difference of the frequencies of the first and the secondacoustic waves, and wherein the collimated beam has a frequency of atleast 20 kHz; and a steering device arranged to direct the collimatedbeam in at least two directions into the material around the borehole,wherein the at least two directions include an azimuthal directionaround a longitudinal axis of the borehole and an inclination withrespect to the longitudinal axis of the borehole; and one or morereceivers arranged to receive the collimated beam after it has reflectedor backscattered from an inhomogeneity in the material around theborehole.
 23. The system in accordance with claim 22, wherein thecollimated beam of at least 20 kHz has a frequency range.
 24. The systemin accordance with claim 23, wherein the frequency range of thecollimated beam is between 20 kHz and 120 kHz.
 25. The system inaccordance with claim 22, wherein the length of the nonlinear materialis between 5 cm and 2 m.
 26. The system in accordance with claim 22,wherein the non-linear material is selected from the group consistingof: a mixture of liquids, a solid, a granular material, embeddedmicrospheres, an emulsion, and combinations thereof.